Rounded Stroke Bug with Intuos4 and Intuos5 tablets
See if you have this bug, I tested it on 2 computers and I noticed the same behavior.
By default the click threshold is a bit low, so you need to change it, but watch how it reacts in Photoshop. The sensitivity is rather bad. Worse is if you try modifying your grip pen preferences it changes the quality of the line.
This affects the intuos4 and intuos5 tablets, but I suspect any other device like the Cintiqs with 2048 levels of pressure sensitivity.